The title octa­deca­trienoic acid derivative, C18H26O4, was isolated from Silene maritima With. (Caryophyllaceae), the first time this natural compound has been found in the Caryophyllales order. This fatty acid has an 18-carbon backbone with three double bonds on trans (E) conformation and two carbonyl. In the crystal, molecules are linked via pairs of O—H⋯O hydrogen bonds, forming inversion dimers.

Silene maritima With. belongs to the Caryophyllaceae family (Bremer et al., 2009) and is a perennial species found on cliffs and shingle beaches in coastal habitats (Baker, 1978). This species is known to be a heavy-metal indicator (Price & Abrahams, 1994), and the only phytochemical investigation previously carried out on its aerial parts has revealed the presence of glycinebetaine, a compound used by cells for protection against osmotic stress (Adrian-Romero et al., 1998).

This study is the first report of the presence of 9,16-dioxo-10E,12E,14E octadecatrienoic acid in the Caryophyllales order. This compound has previously been described only in the Asteraceae (Herz & Kulanthaivel, 1984) and Lamiaceae (Li et al., 2011) families.

Its molecular structure contains an 18-carbon backbone with three double bonds in the trans conformation and two carbonyls (Fig. 1). The existence of intermolecular hydrogen interactions between two carboxylic functions was also observed (Fig. 2). The structure of this fatty acid might involve a lipoxygenase action on the α-linoleic acid (Vellosillo et al., 2007). Thus suggesting that it could belong to oxylipines, a class of compounds implicated in environmental stress responses (Browse, 2005; Schaller et al., 2004; Wasternack, 2007).

For background to Silene maritima With., see: Baker (1978); Bremer et al. (2009). This species is known to be a heavy-metal indicator (Price & Abrahams, 1994), and phytochemical investigation on its aerial parts revealed the presence of glycinebetaine, a compound used by cells for protection against osmotic stress (Adrian-Romero et al., 1998). The title compound has previously been described in the Asteraceae (Herz & Kulanthaivel, 1984) and Lamiaceae (Li et al., 2011) families. The structure might involve lipoxygenase action on the α-linoleic acid (Vellosillo et al., 2007), thus suggesting that it could belong to oxylipines, a class of compounds implicated in environmental stress responses (Browse, 2005; Schaller et al., 2004; Wasternack, 2007).

The sampling station is situated in the littoral zone of the western coast of Brittany (Brélès 29, France). Sampling was carried out in July 2008. The aerial parts of the plant were collected, air-dried, and grinded into a fine powder using a grinder (Retsch, ZM 200). Hydroalcoholic extract of aerial parts (1 kg) was prepared by soaking it at room temperature in 3 x 10 l of EtOH/H2O (6/4, v/v) during first 14 h, then 4 h and again 4 h, until exhaustion of raw materials. The extract was then filtered and dried under vacuum using a rotavapor. The amorphous solid, a black-brownish mass, was then dissolved in d-H2O and extracted sequentially with cyclohexane, CH2Cl2, AcOEt and n-BuOH. The CH2Cl2 extract (2.496 g) was fractionated on a silica gel column (SI60 0.050–0.16 mm in size, Merck) eluted successively with cyclohexane (500 ml), AcOEt (1170 ml) and MeOH (330 ml) to yield five main fractions. The third fraction (210 mg) was re-dissolved in MeOH and subjected to semi-preparative HPLC purification (Gilson, binary solvent system). The isolation was performed with a reverse phase Nucleodur C18 ec (250 mm x 21 mm, 5 µm) from Macherey-Nagel. Eluent A was H2O with 0.01% HCOOH, and eluent B was ACN. The flow rate was 10 ml/min and the injection volume was 400 µl at 40 mg ml-1. The elution conditions applied were: 0–5 min, linear gradient from 10% to 15% B; 5–55 min, 15% to 65% B; 55–60 min, 65% to 100% B; 60–70 min, 100% B isocratic. Simultaneous UV monitoring was set at 316 nm. This experimental procedure allowed us to isolate the title compound C18H26O4 at the retention time of 26 min. The pure compound (1 mg) was re-dissolved in 0.2 ml of MeOH/CHCl3 (2/1). The corresponding crystals of 9,16-dioxo-10E,12E,14E octadecatrienoic acid were grown thanks to a slow solubility decrease during two weeks at room temperature after addition of n-heptane (0.4 ml).

The H atoms, except for the H-atom of the carboxyl group which was located from Fourier difference maps, were positioned geometrically and refined using a riding model, with C—H = 0.95–0.99 Å and with Uiso(H) = 1.2 (1.5 for methyl groups) times Ueq(C).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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